1. Field of the Invention
The present invention is directed to a computed tomography apparatus, and in particular to a computed tomography apparatus which undertakes a spiral scan.
2. Description of the Prior Art
In x-ray computed tomography (CT), the conventional standard registration of individual slices is being replaced to an increasing degree by continuous volume exposures using spiral CT techniques. This type of scanning is also referred to as helical scanning, however, the term "spiral scanning" will be used herein. In conducting a spiral scan, a relative displacement along the z-axis is undertaken between the examination subject and the plane containing the x-ray fan beam. The x-ray fan beam is simultaneously rotated. This means that, at any given time, there will be only one point of true projection data in the planar image. Projection data for a planar slice are thus obtained by interpolating the spiral data at two points having the same rotational phase on either side of the planar slice. This procedure is repeated for all rotational phases to obtain interpolated data for 360.degree., sufficient to reconstruct a complete planar image. An image at the selected slice position can then be obtained by undertaking conventional image reconstruction.
Different interpolation methods are utilized for the reconstruction. Such different interpolation methods are used in an effort to influence (improve) the slice sensitivity profile and the noise properties of the image. In the attempt to fashion the profile as thin as possible, i.e., to keep the spatial resolution in z-direction optimally high, so-called 180.degree. algorithms are recoursed. The implementation of these methods usually ensues in the form of weighting methods in order to keep the computational outlay as low as possible.
For a given series of images that are calculated with this method, however, it is frequently the case, particularly given large subjects, that the noise patterns and the image sharpness as well are non-uniformly distributed over the subject and the preferred (optimum) alignment of these distributions cyclically changes from image to image or frame to frame, a cycle being the distance that is traversed during a 360.degree. revolution of the x-ray tube. This is illustrated in FIG. 1 on the basis of a phantom. FIG. 1 shows the curve of relative variance of the noise (i.e., normalized .sigma..sub.o.sup.2 dependent on the spiral weighting w (.theta.)). The non-uniform distribution is considered disturbing and can impede the examination of the imaged slice; given threshold-based, three-dimensional presentations of the image data sets, it can lead to further artifacts. The degree is of different extent dependent on the specific 180.degree. algorithm employed, but occurs in every instance.